DNAs with defined sequences are often required for biomedical research in order to better investigate specific issues. For example, to identify optimized antibodies for specific purposes, gene libraries for antibodies comprising codon triplets for different amino acids at specific key positions are required. Hereby synthetic DNAs are preferred that comprise defined sequence sections and variable sequence sections.
EP 2110435 B1 describes a method for preparing a nucleic acid library comprising a plurality of elements, each element comprising a first, a second and a third nucleotide section. The sequence of the first and the third nucleotide section are identical respectively in the various elements, while the elements of the nucleic acid library differ in the sequence of the second section. The method uses a first at least partially double-stranded oligonucleotide with a single-stranded overhang, the oligonucleotide comprising the sequence of the first or third nucleotide section. The oligonucleotide is connected to an oligonucleotide library. This oligonucleotide library consists of second at least partially double-stranded oligonucleotides each with a complementary overhang. The resulting ligation product is cut and combined with a second oligonucleotide library to a second ligation product. These steps are repeatedly performed until the variable middle part with the desired number of variable codon triplets is fully synthesized. A particular disadvantage of this method is that in each extension step only one codon triplet is added. In addition, each of the first ligation products from the first oligonucleotide library must be combined with each oligonucleotide of the second oligonucleotide library. This requires a lot of equipment and computational effort.